Selection, Sizing and Analysis Tools for Centrifugal Compressors

Volume Ratio Effect in Multistage Centrifugal Compressors

In a multistage compressor, changes in molecular weight, temperature, and/or rotative speed can change the stage which initiates surge. This is due to the volume ratio effect, and changes the shape of the surge line.

The attached animation shows the performance operating envelope of a 5 impeller barrel-type compressor; it provides a demonstration of the volume ratio effect on the shape of performance map due to:

- Increase of rotating speed (0'.13''): As the rotating speed increases and approaches design speed (~90%), the stage that initiates surge transitions in a progressive manner from the 1st stage to 5th (last) stage. At approximatively 103% speed, the same stage (5th) restricts the map from surge and choke sides. At higher speeds (exceeding 103% speed), the stage responsible for restricting the operating envelope in the vicinity of the stonewall region transitions from 5th (last) to 1st stage.

- Change in gas composition and molecular weight (0'.38''): With molecular weight increase, the volume ratio effect is triggered at lower speeds (~60%), at which the Mach number is approximately equivalent to design one. The narrowing of the performance envelope and available stability and turndown at speeds near design and higher is substantial. Not accounting for this effect can result in substantial misestimation of the real range actually available for operation.

- Change in inlet temperature (0'.58''): The animation below shows how the surge and choke lines as well as the shape of the performance curves are affected by an increase of inlet temperature from 300 K to 360 K.

In order to obtain the overall performance map of the compressor, a so-called stage stacking procedure is used by the performance prediction program. Each stage performance is modeled individually by means of an artificial neural network; the calculation then determines the shape and limits of the overall performance envelope.

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